I'm trying to piece together a lighting system for growing plants in sterile tissue culture; LED illumination is ideal, as we don't need the intensity used in other settings (for growing mature plants, for example).

I've decided on going with Osram Golden Dragon Plus LEDs, using two different models- one peaking in the red, one peaking in the blue. Reds run 2.15 volts, and I'm going to run them in a series of 9; at 700 mA each, that works out to about 14 watts (according to the calculator at http://led.linear1.org/led.wiz).

The other string is blue; they run 3.2 volts, and I want 1/3 as many as reds; so, with 3 blue LEDs, running at 400 mA, consumption is 4 watts. The problem is, they're at a separate voltage (10 volts).

Last night, I thought I had a solution using the website above using 20 volts for both, but Firefox crashed and I lost everything. I can't figure out how to run both strings off of 20 volts without huge resistors- and lots of wasted power.

My questions:

1) Is there any sort of way to figure out how to optimize efficiency without plugging in these numbers manually- maybe a spreadsheet? Or can someone else tell me how to optimize the values for 20 volts on both strings?

2) If I can't use separate voltage on each string, can I use a 20 volt power supply, and then a zener diode (or perhaps something else?) to get 10 volts for the other string?

That wizard is kinda bogus. It uses "Washington" math. The red string with a forward voltage of 2.15V each is going to eat up 19.35V. With a 20V source, that leaves no headroom. You'll be better off going with a string of 8 LEDs. A forward voltage of 2.15V at 700mA probably makes the LEDs 1.5W LEDs. Can't really change that. With 8 of them, you're gonna have ~12W dissipated through the string, no matter how you hook them up, unless of course you run them at a lower current.

Anyways, 8 diodes at 2.15V each is gonna take 17.2V. 700mA in a string, if you were going to use a resistor for this, you'd be looking at a value of about 4Ω which would dissipate 2W, so you'd want a resistor that was larger than a 2W resistor. I don't think I'd recommend this, of course, I'm no expert, but I've been around the LED block, especially here recently, and I think a constant current source would be far more suitable and reliable for this.

More than likely, you'll be looking at needing heat sinking on the diodes and some current source components as well. Somebody else can probably tell you a lot more about current sources than I can, since they are still new to me, and I don't want to steer you in the wrong direction. However, an internet search for "constant current source" or "LED driver" will turn up lots of info that you can look over in the meantime.

Since you have some experience there are ways around your dilemma, if you are willing to put some work into it.

The easiest is to google "buck puck", you can buy off the shelf SMPS current regulators. They will not get hot, are extremely efficient (figure around 80% conversion), and are easy to use. The down side is they are relatively expensive, $10-15 each, and you would need one with each LED chain.

20V is a good number, I like 24V better. With buck pucks you may need smaller voltages, but with commercial units maybe not.

If you want to make one it isn't too hard, just tedious.

I've helped quite a few folks with grow lights, generally for beginners the linear option is the simplest. It would be interesting to help someone with a truly efficient system for a change. Are you interested in going through the exercise?

At LEDSupply.com the buckpucks run $14.97 each or $17.97, depending on if they are wired or pinned. They don't have a 400mA version, but do have a 350mA and 700mA version. This type of current regulation is critical to the life of your LED's as is proper heat sinking. A 24V power supply of 1.5A - 2.0A would be sufficient. The buckpucks take a maximum of 32V, but would not be a voltage you would want to keep them at. 24V is perfect.

If they would settle for 8 reds and 3 blues, then 12v would give sufficient headroom aka "wiggle room" for a buck-type regulator; 3.4v for the reds, and 2.4v for the blues. The BuckPucks need a minimum of 2v headroom.

Three BuckPucks will set one back around $50. There are much less expensive options than a BuckPuck, but it requires assembly of several components to make a switching buck regulator for each string of LEDs.